Imaging studies are the cornerstone for tumor localization in patients with Cushing's syndrome caused by ectopic adrenocorticotropin hormone (ACTH) secretion (EAS). Computed tomography (CT) and magnetic resonance imaging (MRI) are used most commonly to localize the source of EAS. However, in 30-50 percent of patients with EAS the source of ACTH secretion cannot be found despite repeated studies over time. Up to half of these patients do not respond to medical therapy of hypercortisolism and must undergo bilateral adrenalectomy with lifelong replacement therapy. Thus, there is a need for improved imaging techniques to identify ACTH-secreting tumors.[unreadable] [unreadable] Nuclear medicine techniques enable in vivo imaging of physiological and pathophysiological processes, and among these techniques, positron emission tomography (PET) studies are increasingly used in oncology. We previously evaluated the utility of 18F-fluorodeoxyglucose (FDG) positron emission tomography (PET) or 111In-DTPA-D-Phe-pentetreotide (OCT) at higher than standard doses of radionuclide (18 mCi; H-OCT), and found that FDG-PET did not detect tumors that were occult on CT/MRI. H-OCT rarely identified a lesion. Thus, conventional modalities of CT and MRI should be used to image the neck, thorax, and abdomen in these patients. FDG-PET does not provide additional information. H-OCT may be useful when other imaging modalities fail to localize the ACTH-secreting tumor. [unreadable] [unreadable] Currently we are extending these studies to evaluate the utility of 18F- L-3,4-dihydroxyphenylalanine (18F-DOPA) PET to identify these tumors. This compound is a precursor for serotonin production in neuroendocrine tumors, and thus is a good candidate for PET examination since most occult ACTH-secreting tumors are neuroendocrine. [unreadable] [unreadable] Measurement of plasma ACTH levels by RIA is used to identify adrenal causes of Cushing's syndrome and to distinguish ectopic Cushings syndrome from Cushing's disease. We wished to determine whether diagnostic criteria developed with RIA would also be applicable for immunoradiometric (IRMA) or immunochemiluminescent (ICMA) assays. ACTH was measured by RIA, IRMA and/or ICMA assay on samples obtained during basal sampling, following CRH and during IPSS. The primary outcome was comparison of diagnostic utility. IRMA results, while lower, correlated highly with RIA (r=0.9, p<0.0001) and had similar sensitivity (100 vs 80%) and specificity (89 vs 94%) for the diagnosis of adrenal Cushings syndome (p=0.3) and for the diagnosis of Cushings disease during CRH testing (85 vs 83%, p=1.0). During IPSS, IRMA had similar sensitivity (100%) and specificity (100 vs 83%) compared with ICMA or RIA (p=1.0). Thus, this study supports the use of ACTH immunometric assays, which are more available than RIA and offer similar diagnostic utility.[unreadable] [unreadable] Partial or total removal of the pituitary cures 60-80% of patients with Cushing's disease (CD) in whom an adenoma cannot be identified at surgery. Many patients who fail complete or partial hypophysectomy are cured by sellar and parasellar irradiation. As part of a series of prospective studies of CD, we identified 12 patients (34.5 +/- 19.9 yr; 11 females; four children) with tumors located completely within the neurohypophysis among 730 patients undergoing surgery for CD. Tumor was visible at surgery in 11 patients; all 12 tumors were positive for ACTH by immunohistochemistry. Two tumors were excised at repeat surgery because of persistent hypercortisolism within 14 d of negative exploration of the adenohypophysis. There were no long-term complications. At follow-up of 71.9 +/- 34.2 months (range, 30-138 months), all patients were in remission. Adult patients had significant improvement in weight and body mass indices, with restoration of normal menses in all women. In the four pediatric patients, height, weight, and body mass indices were restored toward normal by surgical remission of CD. Hypopituitarism or long-term neurohypophysial dysfunction did not occur. This represents a new subset of patients with CD, ACTH-secreting adenomas that arise wholly within the posterior lobe of the pituitary gland. In cases of CD in which an adenoma is not identified in the adenohypophysis and in patients with persistent hypercortisolism after complete or partial excision of the anterior lobe, tumor within the neurohypophysis should be considered; selective adenomectomy of a neurohypophyseal, ACTH-secreting tumor can produce long-term remission.